The invention relates to improvements in fluid flow machines, and more particularly to improvements in casings for fluid flow machines such as single-stage or multi-stage centrifugal pumps. Still more particularly, the invention relates to improvements in fluid flow machines of the type wherein the casing includes a first portion or section serving to convey a stream of fluid substantially at right angles to the axis of the shaft which drives one or more fluid conveying elements or which receives torque from one or more fluid conveying elements.
Many types of fluid flow machines are provided with casings wherein a first portion constitutes or resembles a tube serving to direct a stream of fluid radially inwardly toward the axis of the shaft and the outlet of such first portion admits the fluid into a passage which surrounds the shaft and serves to direct the admitted fluid into the range of the foremost impeller if the machine is a multi-stage pump. Thus, the direction of fluid flow is changed through an angle of approximately 90 degrees because the fluid is supplied radially inwardly toward the shaft and is then caused to flow in parallelism with the shaft. Such pronounced change of the direction of fluid flow often results in the development of a highly unpredictable stream in the region immediately ahead of the foremost fluid conveying element on the shaft. The stream is likely to cause cavitation which entails damage to the foremost fluid conveying element and thus reduces the life span of the affected element.
German Auslegeschrift No. 21 58 518 discloses a multi-stage centrifugal pump. FIGS. 6 and 7 of this publication show two forms of a casing with an inlet portion and a portion which defines a passage for the flow of fluid from the inlet portion to the foremost impeller. The pump casing is formed with an internal surface having a U-shaped or crescent-shaped outline. The casing is also formed with three or four bores which convey fluid from a location ahead of the outlet of the radially extending inlet portion so as to direct streamlets of fluid directly into the annular passage around the shaft along the full width of the internal surface, as seen in the axial direction of the shaft. The numerous streamlets which are diverted from the main stream of inflowing fluid produce extensive turbulence in the body of fluid filling the passage with the well known undesirable effects upon the performance and useful life of the impellers.